Photolithography is commonly used to make miniaturized electronic components such as integrated circuits in semiconductor manufacturing. In a photolithography process, a layer of photoresist is deposited on a substrate, such as a silicon wafer. The substrate is baked to remove any solvent remained in the photoresist layer. The photoresist is then selectively exposed through a photomask with a desired pattern to a source of actinic radiation. The radiation exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer solution to remove either the exposed portions of the photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The patterned photoresist can then be used as a mask for subsequent fabrication processes on the substrate, such as deposition, etching, or ion implantation processes.
In a photolithography process, dense patterns generally tend to have better process windows than isolated or semi-isolated patterns due to proximity effects. One approach to improve the process windows of isolated or semi-isolated features is to use sub-resolution assist features (SRAFs) in a single exposure process. Extensive simulation and modeling are needed to put SRAFs in a photomask. Manufacturing such photomasks is also complex and expensive. In addition, it is difficult to introduce partially transparent patterns at a specific complex pattern area on a photomask. Another approach is to use printed sub-resolution assist features (PRAFs) in a double exposure double etch process. However, in general, the double exposure double etch process is more costly than the single exposure process. Thus, there is a need to improve the process windows of isolated and semi-isolated patterns.